Reactor and Process for the Hydrogenation of Carbon Dioxide

ABSTRACT

The present invention is directed to a membrane reactor for the hydrogenation of carbon dioxide, said membrane reactor comprising a reaction compartment (2) comprising a catalyst bed, a permeate compartment (4) and a membrane separating the reaction compartment and the permeate compartment, wherein said permeate compartment comprises a condensing surface.

The invention is in the field of chemical reactors and chemicalprocesses. In particular, the invention is directed to a reactor and aprocess for the hydrogenation of carbon dioxide into methanol and/ordimethyl ether.

The process of hydrogenating carbon dioxide into methanol and/ordimethyl ether (also referred to as DME) can i.a. be used to lower CO₂concentration in the atmosphere. In addition, the process can also beused to further valorize biogas and to provide a broader applicabilityof said biogas. For instance, by separating biogas into CO₂ and methane,the methane can be reformed into hydrogen (H₂) and the obtained hydrogencan be allowed to react with the CO₂ to form methanol and/or DME. Thisconcept is not limited to biogas since in principle any CO₂-containinggas can be valorized and provided with a broader applicability byhydrogenating the CO₂ therein.

The hydrogenation of CO₂ is believed to involve the following reactions:

CO₂+3H₂

CH₃OH+H₂O  (I)

CO₂+H₂

CO+H₂O  (II)

CO+2H₂

CH₃OH  (III)

2CH₃OH

CH₃OCH₃+H₂O  (IV)

The reaction CO₂ to MeOH can proceed directly (equation I), or via theintermediate CO (equation II and III). Depending on the processparameters, the obtained MeOH can react further into DME (equation IV).As is apparent from equations I-IV, water is produced as a side productconcomitantly with MeOH and/or DME. In addition, since these reactions(I-IV) are in equilibrium under the process conditions, full conversionto the products can in principle not be obtained under thermodynamicequilibrium conditions. Therefore, in order to maximize the conversioninto methanol and/or DME, it is preferred that at least one of thereaction products which is formed in the reaction (i.e. water, methanoland/or DME) is removed from the reaction medium.

Examples of conventional methods for the removal of the reactionproducts include condensation in a recycle configuration, reactiveadsorption of water, and in-situ water-consuming reaction. US2004/064002 discloses water vapor permeation through a membrane toremove the water from a reaction of MeOH to DME. In WO2015/030578, areactor and a process are described comprising two stages including azone or stage wherein a liquid condensate of methanol and water iscondensed. However, all these methods are associated with severaldrawbacks.

For instance, a recycle configuration results in a bulky process designwith installation of separate equipment, piping and control, while theuse of solid system for the adsorption or reactive-consumption of waterpose challenging regeneration and handling, also making the overallprocess bulky and expensive. Furthermore, a solid adsorption systeminvolves regeneration which leads to a discontinuous type of operationand higher temperatures are needed for regeneration. Although the use ofwater vapor permeation through a membrane is elegant, this conventionalmethod is associated with concomitant removal of H₂ from the reactionand decrease of the overall conversion. The combined condensation ofmethanol and water as disclosed in WO2015/0305578 requires a subsequentseparation of the water and methanol in order to obtain dry methanol andresults in a liquid methanol product, whereas it may be preferred tomaintain a gaseous methanol product if is going to sequentially beconverted into products in a gaseous phase reaction. This process istherefore relative high in energy consumption.

It is an object of the present invention to provide a process andreactor, in particular for the hydrogenation of carbon dioxide, thataddresses at least one of these drawbacks.

A further object of the present invention is to provide an improvedhydrogenation of carbon dioxide, by enhancing reaction rates towards theproduction of methanol and/or DME, improving catalyst lifetime,improving reactor design and intensification of process of production ofmethanol and/or DME from several steps to singlereaction-separation-removal unit, and/or by providing a conversionbeyond equilibrium constraints due to selective removal of at least onereaction product from the reaction mixture.

The present inventors found that these objects can at least partially bemet by a combination of permeation of the formed water through amembrane and condensation of said water after permeation. This providesa method to selectively remove the water from the process, without therequirement of bulky process design with installation of separateequipment. The permeation and condensation are carried out in the samereactor, which reactor is another aspect of the present invention (videinfra). In particular, this method enables the in-situ removal ofcondensed water from the reaction within the same reactor in which thehydrogenation reaction is carried out.

Accordingly, the present invention is directed to a process for thehydrogenation of carbon dioxide, wherein said process comprises reactingcarbon dioxide with hydrogen to form methanol and/or dimethyl ether, andwater as a side product, and wherein said process further comprisesremoving said water from the process by a combination of permeation ofsaid water through a membrane and condensation of the water.

In another aspect, the present invention is directed to a membranereactor for the hydrogenation of carbon dioxide, said membrane reactorcomprising a reaction compartment comprising a catalyst bed, a permeatecompartment, and a water-permeable membrane that separates the reactioncompartment and the permeate compartment. Said permeate compartmentcomprises a condensing surface such that the reactor is particularlysuitable for carrying out the method of the present invention. Thecondensing surface in the permeate compartment is configured and isconnected to a means for cooling (herein also referred to as coolingmeans) such that during operation of the reactor, the water can condenseon the surface.

The means for cooling may be such to effect active and/or passivecooling. Examples of passive cooling include heat sinks and heatconductive materials that can be passively cooled by conductive coolingdue to a temperature difference. Active cooling however is typicallymore effective and therefore preferred. Active cooling can be obtainedby providing the means for cooling with an active-cooling device, forinstance a passage such as a tube through which a cooling fluid can flowand/or a fan directing air. Suitable cooling fluids include gases (e.g.air) and liquids (e.g. water). A combination of active and passivecooling, e.g. by providing both fluid cooling through a tube or by a fanand a heat sink can also be used.

The water-permeable membrane is typically not exclusively permeable forwater. For instance, hydrogen molecules, which are also present in thereaction compartment, are smaller than water molecules and may also(albeit undesirably) permeate the membrane. In fact, due to thispermeation of hydrogen, substantial amounts of hydrogen removal from thereaction mixture have been observed in the aforementioned conventionalmethods. It is believed that the requirement of a sweeping gas stream toremove the vapor, also removes the permeated hydrogen. Advantageously,the presence of the condensing surface mitigates the requirement of thesweeping gas stream and the condensing surface selectively condensateswater while hydrogen remains gaseous under the condensation and reactionconditions. With this method, separation of water from the reactionmixture and from 112, may be carried out in one step, limiting the needof additional equipment for individual steps of separation andpurification.

Typical reaction conditions include elevated temperatures and pressures.In particular, reacting carbon dioxide with hydrogen may be carried outat a temperature in the range of 150-400° C., preferably in the range of200 to 300° C., more preferably about 250° C. and/or at a pressure inthe range of 1-10 MPa, preferably in the range of 2-8 MPa, morepreferably about 5 MPa.

The water-permeable membrane (herein also referred to as the membrane)can accordingly suitably function at these reaction conditions.Therefore, the water-permeable membrane is preferably a hydrophilicmembrane, which preferably comprises a zeolite membrane, an amorphousmembrane or a polymer membrane. Particularly suitable zeolite membranescomprise mordenite (MOR), ZSM-5, chabazite (CHA), silicalite-1 (SIL-1),Z4A, faujasite (FAU), Si/Al variant MFI, and the like. Suitableamorphous membranes may comprise Al₂O₃/SiO₂. The polymer membranes maycomprise ceramic-supported polymers (CSP), as these are particularlysuitable for functioning at elevated temperatures. Membranes that areused for the water removal in Fischer-Tropsch processes may also besuitably used for the present invention (see e.g. Rohde et al.Microporous and Mesoporous Materials 115 (2008) 123-136).

The catalyst bed may comprise any known catalyst that suitably catalyzesthe hydrogenation of carbon dioxide (see e.g. Gallucci et al. ChemicalEngineering and Processing 43 (2004) 1029-1036 and references therein,which are incorporated herein in their entirety). Particularly suitablecatalysts comprise copper, zinc oxide, zirconia, palladium, cerium(IV)oxide or combinations thereof. The catalyst may be supported on suitablesupports such as alumina or silica. Advantageously, the presentinvention can increase the lifetime of the catalyst due to an increasedremoval of the water. As such, less water may condense in the catalystparticles, which can increase both performance and lifetime.

In FIG. 1, a particular embodiment of the reactor in accordance with thepresent invention is illustrated. The reactor comprises:

-   -   an inner wall (5) bounding an inner space that defines the        reaction compartment (2);    -   an outer wall (7) that is arranged around said inner wall,        wherein said outer wall (7) and inner wall (5) bound an outer        space that defines the permeate compartment (4);        wherein said inner wall (5) comprises the water-permeable        membrane (51). Preferably, as illustrated in FIG. 1, the inner        wall and outer wall are tubular and the outer wall is co-axially        arranged around said inner wall.

In a preferred embodiment, the condensing surface is part of aprotruding surface elements (62) that are connected to or part of thecooling means. In a particular embodiment, the protruding surfaceelements are connected to said inner wall (5) and protruding into thepermeate compartment. An example of this embodiment is illustrated inFIG. 2. In an alternative, embodiment, the protruding surface isconnected to outer wall (7) and protruding into the permeate compartment(not shown). This may be preferred as condensation of water in themembrane is preferably prevented because condensed water in the membranecan block the membrane' pores. The protruding surface elements and thusthe surface can have various shapes. For instance, the surface may beundulating or jagged to increase the total surface area. Suitable shapesmay result from protruding elements such as rods, bars, blades and thelike. Such protruding elements may comprise at least part of theactive-cooling device such a tubing.

In particular embodiments, the condensing surface is situated in thereactor in the vicinity of the membrane. As such, the permeated watercan travel a relatively short path before it condenses. The inventorsfound however, that in certain embodiments, it may actually be preferredthat the cooling means and the condensing surface is situated in thereactor away from the membrane. For the hydrogenation reaction, typicalreaction temperatures are around 250° C. due to which the membrane mayhave a temperature of about 200° C., while it is preferred that thecondensing surface is kept at a much lower temperature (vide infra).Accordingly, such a temperature difference can more easily andefficiently be maintained when the cooling means and condensing surfaceare situated away from the membrane, or at least by situating thecooling means and condensing surface disconnected from the membrane andthe inner wall. In addition, it can preferred condensed water in themembrane's pores. Away from the inner wall and in the vicinity of theouter wall herein means that the cooling means are closes to the outerwall than to the inner wall.

In FIG. 3, an embodiment of the reactor is shown, wherein theconfiguration is similar to that as illustrated in FIG. 1, but the meansfor cooling (6) in this embodiment is situated away from the inner wall(5) and more in the vicinity of the outer wall (7).

In FIG. 4, yet another configuration of the membrane reactor isillustrated. In this particular embodiment, which is preferred, themembrane reactor (1) comprises

-   -   an inner wall (5) bounding an inner space that defines the        permeate compartment (4);    -   an outer wall (7) that is arranged around said inner wall,        wherein said outer wall (7) and inner wall (5) bound an outer        space that defines the reaction compartment (2);

wherein said inner wall (5) comprises the water-permeable membrane (51).Thus, the embodiment wherein the permeate compartment is at leastpartially enclosed by reaction compartment, is an inverse configurationof the reactor illustrated in FIGS. 1 and 3 wherein the reactorcompartment is at least partially enclosed by the permeate compartment.As further illustrated in FIG. 4, preferably the cooling means (6) andthe condensing surface (61) are located away from the inner wall. Forinstance, in an co-axial, tubular configuration of the reactor, thecooling means can be placed in the (longitudinal) axis of the reactor,as illustrated in FIG. 4 as well. The cooling means however, can havevarious shapes, and are not confined to a straight configuration asillustrated in FIG. 4. For instance, the cooling means can be aU-shaped, helical shaped, jagged and/or undulated tube, a variousthereof. The cooling means can also comprise the protruding elementssuch as rods, bars, blades and the like.

The condensing surface is generally preferably actively cooled, meaningthat it is maintained at a temperature by which water can condensateunder the, generally elevated, reaction pressures. The condensingsurface can be cooled actively by providing a cooling fluid stream, forinstance air or (relatively) cool water, within tubing or a space nearthe condensing surface. It is to be understood that the cooling fluidshould preferably be prevented from contacting the reactants in thereactor, such a hydrogen. The cooling fluid therefore is typicallyseparated from the reaction compartment and the permeate compartment bya wall. For effective condensation under the reaction conditions, thecondensing surface is preferably maintained at a temperature in therange of 50 to 150° C. The inventors however found that water canparticularly effectively be condensed at less than 100° C., preferablyless than 50° C.

Surprisingly, the inventors found that in case the condensing surface is10° C. or less, condensation of the water occurs in such an effectivemanner, that a water mass flux across the membrane can be achieved (seeFIG. 10). The water mass flux across the membrane results in even betterDME yields. For this reason, it is preferred that the cooling means ofthe reactor described herein, are adapted to be able to cool, preferablyactively cool, the condensing surface to a temperature of less then 100°C., preferably less than 50° C., most preferably less than 10° C.,during operation of the reactor wherein a reaction is carried out at atemperature in the range of 150-400° C., preferably in the range of 200to 300° C., more preferably about 250° C.

The condensed water can be collected and let out the permeatecompartment.

For the purpose of clarity and a concise description features aredescribed herein as part of the same or separate embodiments, however,it will be appreciated that the scope of the invention may includeembodiments having combinations of all or some of the featuresdescribed. The invention can be illustrated by the followingnon-limiting examples.

EXAMPLE 1—EFFECT OF WATER REMOVAL ON DME PRODUCTION

The following reaction and reactor is analysed in silico. In a membranereactor as illustrated in FIG. 5, at the feed side (i.e. in the reactioncompartment) a carbon dioxide hydrogenation reaction takes place toproduce methanol and subsequent conversion to dimethyl ether, at 250° C.and 50 bar, according to the reactions 1-4. Reaction 3 is thecombination of reaction 1 and 2. By removing water from reaction 4, thereaction can be shifted to the right-hand side to produce more dimethylether. Therefore, in-situ removal of water from the mixture of H₂, H₂O,CO, CH₃OH, CH₃OCH₃ will drive the reactions towards more DME production.To show the effect of water removal, two models were created.

CO+2H₂↔CH₃OH  (1)

CO₂+H₂↔CO+H₂O  (2)

CO₂+3H₂↔CH₃OH+H₂O  (3)

2CH₃OH ↔CH₃OCH₃+H₂O  (4)

In the first model kinetic equations and equilibrium constants forreaction 1-3 were used from Portha et al., Erena et al. and Alharbi etal., to show the establishment of chemical equilibrium in DMEproduction. (see also Portha et al., Ind. Eng. Chem. Res., 56 (2017)13133-13145, Ereña et al., Chem. Eng. J. 174 (2011) 660-667 and Alharbiet al. ACS Catal 5 (2015) 7186-7193. This equilibrium limits the amountof DME produced. By modelling the chemical equilibrium with and withoutwater removal, it is shown that in-situ water removal with membranereactor leads to an increase in DME production. Table 1 contains thestarting conditions and pressures for the equilibrium model.

TABLE 1 Input values for equilibrium model Input values: T [° C.] 250 P[bar] 50 Start pressure CO₂ [bar] 7.5 Start pressure CO [bar] 7.5 Startpressure H₂ [bar] 35

A second model calculates the steady state water removal by the membranereactor, driven by the pressure difference between feed and permeateside. The calculated water removal from the feed side is used as aninput for the first model in the graph, to show the increase in DMEproduction. Additionally, variations in air gap and temperature of thecooling element show the effect of different parameter on the membranereactor's performance

For modelling a steady state in-situ water removal during conversionfrom CO₂ to dimethyl ether (DME), theory was used that is commonlyapplied for air gap membrane distillation processes used in watertreatment. FIG. 6 shows the temperature and pressure profiles for themembrane reactor from FIG. 5. Water removal through the membrane isdriven by a pressure difference between the water gas pressure on thefeed side P_(F) and the water pressure just above the liquid film on thecondenser surface P_(C).

The water gas pressure on the feed side is calculated by multiplying itscalculated vapor fraction with the total pressure on the feed side ofthe reaction.

P_(F) =y _(i)*P_(total)  (5)

On the other side of the membrane, the vapor pressure just above theliquid film on the condenser surface is described by the Antoineequation (see equation 6).

$\begin{matrix}{P_{C} = 10^{4.6543 - {\frac{1435.264}{T_{cool} - 64.848}{({{1.0E} + 5})}}}} & (6)\end{matrix}$

The dominant mechanism for the water vapor mass flux is indicated by theKnudsen number. Equation 7 is used to calculate the Knudsen number, withk_(B),T,r, d_(H2O) and P as Boltzmann constant, temperature, membranethickness, diameter water molecule and pressure.

$\begin{matrix}{K_{n} = \frac{k_{B}T}{2L\; \pi \; d_{H\; 2O}^{2}P\left. \sqrt{}2 \right.}} & (7)\end{matrix}$

A membrane thickness of 1 mm gives a Knudsen number smaller than 0.01indicating that molecular diffusion through the air gap will be the masstransfer limiting step at 250° C. and 50 bar. Molecular diffusionthrough the air gap is usually the limitation in mass transfer in airgap membrane distillation.

Equation 8 gives the water flux for transition flow. Based on membraneproperties in Table 2, a flux of 0.097 kg/m²/s was calculated.

$\begin{matrix}{J = \frac{ɛ\; {PDM}\; \Delta \; p}{\left( {{\tau\delta} + b} \right){{RT}\left( P_{a} \right)}}} & (8)\end{matrix}$

TABLE 2 properties used in flux calculations Properties Values used incalculation Temperature condenser surface [° C.] 8 Radius condenserelement [mm] 1 Air gap [mm] 2 Vapor fraction H₂O in feed gases yi2.94E−4 Length membrane [m] 0.1 Membrane thickness [mm] 1 Membrane poreradius r [μm] 1 Porosity ε 0.56 Tortuosity τ 2 Diffusion coefficientwater-air [m²/s]  2.8E−5 Flow rate H₂O feed side [l/s] 3

Equation 8 represents the one-dimensional flux, whereas the experimentalsetup is cylindrical. As a simplification, the surface of thecylindrical membrane area was multiplied with the one-dimensional fluxto get the total flux to be deposited on the inner cylindrical coolingelement.

The result of the in silico reaction are illustrated in FIGS. 7 to 9.

FIG. 7 shows the model for establishing chemical equilibrium forreactions 1-4. This represents the chemical reactions that occur on thefeed side of the membrane reactor, without water removal, starting fromthe pressures in Table 1. This can be compared to the situation wherethe cooling element at the permeate side of the membrane reactor is atthe same temperature as the feed side and there is no in-situ waterremoval. Water is produced in reaction 3, but consumed in the water gasshift reaction (reaction 2 to the left hand side). Overall, withoutwater removal, the end concentration of DME is 6.21 vol %.

FIG. 8 shows the same chemical equilibrium model with a constant waterremoval where 33% of the created water is removed. This amount of waterremoval matches with the water flux calculated in the model of thein-situ water removal membrane reactor with the standard parameters inTable 2. An equilibrium model with an added constant water removalcorresponds to the situation where the reactions 2 and 3 shift more tothe right-hand side of the equations due to the water removal, giving ahigher DME yield. With water removal matching to the flux of waterremoval calculated in the membrane reactor model, the yield of DME riseswith 51 vol %. FIG. 9 shows the 51 vol % increase in DME yield due towater removal.

EXAMPLE 2—EFFECT OF CONDENSING SURFACE TEMPERATURE ON THE FLUX OF WATERACROSS THE MEMBRANE

Based on the models described in Example 1, the water mass flux over themembrane can be calculated, depending on the temperature of thecondensing surface. The results are depicted in FIG. 10, from which itcan be deduced that at condensing surface temperatures of >50° C.,essentially no water flux is expected. The operating temperatures ofmembrane is close to 200° C. at which the condensation on/inside themembranes is not taking place. The results further show that activecooling with condensing temperature of <10° C. results in positive fluxfor water flux across the membrane.

1. A membrane reactor for the hydrogenation of carbon dioxide, saidmembrane reactor comprising a reaction compartment comprising a catalystbed, a permeate compartment and a water-permeable membrane separatingthe reaction compartment and the permeate compartment, wherein saidpermeate compartment comprises a condensing surface and a means forcooling that is connected to said condensing surface such that duringoperation of the reactor, water can condense on the condensing surface.2. The membrane reactor in accordance with claim 1, wherein the meansfor cooling comprises an active-cooling device.
 3. The membrane reactorin accordance with claim 1, wherein said reactor comprises an inner wallbounding an inner space that defines the reaction compartment; an outerwall that is arranged around said inner wall, wherein said outer walland inner wall bound an outer space that defines the permeatecompartment; wherein said inner wall comprises the water-permeablemembrane, and wherein said condensing surface is connected to the outerwall.
 4. The membrane reactor in accordance with claim 1, wherein saidreactor comprises an inner wall bounding an inner space that defines thepermeate compartment; an outer wall that is arranged around said innerwall, wherein said outer wall and inner wall bound an outer space thatdefines the reaction compartment; wherein said inner wall comprises thewater-permeable membrane, and wherein the condensing surface is locatedaway from the inner wall.
 5. The membrane reactor in accordance withclaim 1, wherein said permeate compartment and means for coolingcomprise an active-cooling device which active-cooling device isdisconnected from the inner wall.
 6. The membrane reactor in accordancewith claim 3, wherein said inner wall and outer wall are tubular and theouter wall is at least partially co-axially arranged around said innerwall.
 7. The membrane reactor in accordance with claim 1, wherein thecondensing surface comprises protruding surface elements.
 8. Themembrane reactor in accordance with claim 1, wherein saidwater-permeable membrane comprises a hydrophilic membrane.
 9. Themembrane reactor in accordance with claim 1, wherein said catalyst bedcomprises copper, zinc oxide, zirconia, palladium, cerium(IV) oxide orcombinations thereof.
 10. The process for the hydrogenation of carbondioxide, wherein said process comprises reacting carbon dioxide withhydrogen to form methanol and/or dimethyl ether, and water as a sideproduct, and wherein said process further comprises removing said waterfrom the process by a combination of permeation of said water through awater-permeable membrane and condensation of the water.
 11. The processfor the hydrogenation of carbon dioxide carried out in a membranereactor in accordance with claim 1, wherein the condensing surface has atemperature of less than 150° C.
 12. The process in accordance withclaim 11, wherein reacting carbon dioxide with hydrogen is carried outat a temperature in the range of 150-400° C. and/or at a pressure in therange of 1-10 MPa.
 13. The process in accordance with claim 10, whereinsaid carbon dioxide and/or said hydrogen originate from biogas.
 14. Themembrane reactor of claim 1 wherein the means for cooling comprises apassage through which a cooling fluid can flow.
 15. The membrane reactorof claim 1 wherein said water-permeable membrane comprises a zeolitemembrane, an amorphous membrane or a polymer membrane.
 16. The processof claim 11 wherein the condensing surface has a temperature of lessthan 50° C.
 17. The process of claim 11 wherein the condensing surfacehas a temperature of less than 10° C.
 18. The process of claim 12wherein reacting carbon dioxide with hydrogen is carried out at atemperature in the range of 200-300° C. and/or at a pressure in therange of 2-8 MPa.
 19. The process of claim 12 wherein reacting carbondioxide with hydrogen is carried out at a temperature of about 250° C.and/or at a pressure of about 5 MPa.